Automatically controlled windshield wiping systems typically detect the presence of a liquid or a solid coating on the windshield of an automobile. Depending on the type and quantity of coating present on the windshield, the wiper system is activated to clean the windshield, operating in either an intermittent mode, wherein a varying time delay is inserted between consecutive wipes, or a continuous mode, wherein there is no time delay between consecutive wipes.
For example, U.S. Pat. No. 3,947,131, issued to Karl, discloses a windshield soil detector. A glass pane soil indicator is secured to the unexposed surface of the windshield. A light source and a first optical system direct a beam of light through the unexposed surface to the exposed surface at the angle of total reflection. A second optical system is arranged to allow the beam to pass from the windshield to a photometer. A single transmitting device is arranged to produce a signal indicative of the light returned through the windshield to the photometer. Light sent from the light source is conducted through prismatic devices as it enters or leaves the windshield, so as to allow the light to strike the soiled glass surface at the angle of total reflection. In this way, outside light which strikes the windshield from the outside is always broken at an angle into the interior of the glass, such that no bothersome outside light can reach the photometer. Therefore, no compensation must be made for outside light and the detector can be used during daylight and under changing outside light conditions.
U.S. Pat. No. 4,481,450, issued to Watanabe et al., discloses a windshield-mounted system for controlling a vehicle window and the like. The control system includes first means for emitting a beam of radiation into a section of the windshield from the inner surface thereof at such an angle of incidence that the beam reflects off of the outer surface of the windshield. The system also includes second means for detecting and converting the reflected beam into a first signal, and third means for comparing the first signal with a reference value to generate a second signal. The system also includes means, such as a reversible motor, responsive to the second signal for closing the window of the vehicle.
U.S. Pat. No. 4,798,956, issued to Hochstein, discloses a sensing assembly and method for indicating when moisture or other particles have accumulated on a window. An emitter means is included for disposition on the inside of the window for emitting radiant energy to the window on an ingress axis at an incident angle relative to the inside surface. A detector is included for disposition on the inside of the window for detecting radiant energy from the window on an egress axis at a reflection angle relative to the inside surface and equal to the incident angle. Support means support the emitter and detector for spacing the point of intersection of the ingress axis with the inner surface of the window from a point of intersection of the egress axis with the inner surface of the window at a predetermined distance. The sensing assembly also includes limiting means for limiting the field of view of radiant energy from the window entering the detector by allowing to pass only radiant energy substantially parallel with the egress axis within the field of view.